With the wide spread use of cordless telephones in recent years, there are demands for improvement in the cordless telephone and ensured security therein. As a communication system for enhancing security therein, there is FHSS system (hereinafter called “FH system”).
In FH system, the second harmonic modulating frequencies are varied at random and the frequencies are allowed to make hopping so that a sound signal is transmitted and received over a plurality of channels differing in frequency. Accordingly, security in communications can be enhanced by the use of FH system.
According to communications protocol in the US, the number of channels available on FH system is 92 channels. Out of this, 75 channels are used for conversation. Therefore, it is required that 75 channels of good communicating condition be selected from 92 channels for use in conversation. Such a method has so far been in practice to leave out the portion of channels susceptible to a microwave oven and the like (for example, channel 56-channel 72 out of channels 1-75) as spare channels and not to use such spare channels for conversation. Since the number of usable channels is limited, the channels used have been virtually fixed to limited channels.
Thus, in cordless telephones on FH system so far in use, the channels used are virtually fixed. Accordingly, when background noise levels on such channels are relatively high, the S/N ratio of the communicated voice signal is deteriorated. Then, on the receiving end, it sometimes occurs that the sound signal is not accurately demodulated and contents of the conversation are not accurately conveyed.